The system spans 8.5% of the state's public road mileage, but carries over half of the traffic.

[2][3] All other public roads in the state are either inside incorporated places (cities or towns) or are maintained by the county.

[1][2] WSDOT has also defined some spurs that mainly serve to provide full access between intersecting routes.

(Main highways in more populated areas would continue to be entirely under county control, though sometimes built with 50% state aid.)

Six of these highways were east–west crossings of the Cascades; others included a portion of Chuckanut Drive and a road around the west side of the Olympic Peninsula.

[20][21][22] A 1923 restructuring of the system reassigned numbers to almost all the primary state highways,[23] which were soon marked on signs.

[24][25] The final renumbering[26][27] was authorized by law in 1963 and posted in January 1964, when new "sign route" numbers were assigned that matched the inter-state systems and otherwise formed the present grid.

Among the changes were recommending highways serving state parks and ferry terminals be added to the system.